The present invention relates to a pipe of sheet steel or the like, fabricated of a single strip of metal, and which is provided with spirally extending corrugations.
It has long been recognized that pipes such as are used for conduits, drainage and the like are subject to substantial compressive loads, and that they have greater strength if they are corrugated, instead of being of purely cylindrical construction. For example, Pratt et al. U.S. Pat. No. 365,630 discloses the provision of a tube of a suitable metal which is corrugated spirally with a single rib and which extends from one end of the tube to the other. The tube is produced by placing a plain tube upon a spirally corrugated mandrel, which is placed between the centers of a lathe, and a roll of angular corrugations is then forced into the tube, so as to cause it to take the form of the corrugated mandrel. This corrugated tubing had an exterior sinusoidal profile formed by convex portions connected directly to concave portions. Such tubing is disclosed as being used in connection with boilers, radiators, and the like.
Silk U.S. Pat. No. 1,263,340 discloses a spiral sheet metal pipe which is intended to be used for culverts, sewers, and the like, being formed with continuous spirally extending corrugations, with an upstanding spiral seam provided by flanges extending outwardly from the pipe. The disclosed pipe is formed of a single strip of sheet metal which is wound or coiled into spiral form, and having the lateral edges interlocked by the above noted outstanding flange and seam construction. It has a sinusoidal profile.
Hartman U.S. Pat. No. 1,259,233 provides a drain pipe of sheet metal, for roadways, gutters, and the like, made of a strip having circular corrugations, and which is formed into a circle and welded in the circular form, with plural strips being axially joined. The disclosed pipe has a sinusoidal profile.
Nemer U.S. Pat. No. 3,094,147 provides helically wound strip material formed into bendable tubing, useful, for example, as part of an automobile exhaust system. The edges of the strip of which the tubing is formed with rolled edges which are joined into a four-thickness locked seam. The profile is sinusoidal, interspersed with flat portions formed at the four-thickness seams.
Lupke et al. Defensive Publication T103,901 discloses a stiff thin walled plastic pipe of thermoplastic material, the longitudinal cross section profile of the wall comprising a wave form.
Spirally wound steel pipe is widely used for culverts, storm sewers, subdrains, spillways, underpasses and service tunnels. According to Handbook of Steel Drainage and Highway Construction Products, published by American Iron and Steel Institute (Second Edition, 1971), round or circular corrugated steel conduits are in common use for such purposes for medium and high fills, or trenches, and range in diameter from six inches to 21 feet. The corrugations are stated to be "circular arcs connected by tangents" and are described by pitch, depth and inside forming radius. Riveted and resistance spot-welded pipe are noted, having circumferential seams, with the corrugations being of two and two-thirds inch pitch by one and one-half inch depth and three inch pitch by one inch depth. There is also noted lock seam pipes, with the seams and corrugations running helically (or spirally) around the pipe. Small diameters of six, eight, ten inches, etc., have a pitch of one and one-half inches by one-quarter inch depth, while larger sizes, with diameters up to twelve feet, have a two inch pitch by one-half inch depth, two and two-thirds inch pitch by one and one-half inch depth and three inch pitch by one inch depth. Tables are provided for the configurations of the various pipes, with the pipes being of convex and concave circular arcs connected by inclined flat tangential portions. This publication provides tables setting forth the maximum cover of fill material for corrugated steel pipe, taking into account pipe diameter and thickness of the sheet steel material of which the corrugated pipe is made. For example, Table HC-1 provides that with a corrugated pipe having a two and two-thirds inch pitch and one-half inch depth corrugations, for an H 20 Live Load, where the pipe diameter is eighteen inches, and the pipe wall thickness is 0.052 inches, the maximum cover is 132 feet. With other factors being the same, and the thickness increased to 0.079 inches, the maximum cover is 207 feet. With the use of these tables, engineers designing culverts, drain pipes, etc., are able to determine the required size of pipe, based upon such variable factors as the type of load, depth of fill, and type of soil.
As indicated by the above noted Handbook, the corrugated steel pipe which has been produced has been engineered for the maximum fill which pipe of a particular configuration is capable of safely supporting. To this end, the thickness and the shapes of the corrugations have been standardized, the corrugations having, as above noted, circular arcs connected by tangents which were inclined. Such pipes have been sufficiently strong to sustain the design loads as set forth in the above noted Handbook.
Consequently, an engineer, under present practices, has a limited number of available pipe sizes and dimensions. For example, where the engineer is to specify a corrugated steel pipe for an H 20 Live Load, the pipe having an eighteen inch diameter, such pipe is available in thicknesses of 0.052 inches, 0.064 inches, and 0.079 inches, which are specified for maximum cover in feet of 132 feet, 166 feet and 207 feet, respectively. If the engineer is faced with a project which requires such an eighteen inch diameter corrugated steel pipe, with a rated H 20 Live Load, and he is aware that his maximum cover will be, for example, 30 feet, then he is faced with specifying a pipe having a capability of withstanding more than four times the actual cover of that project. Nevertheless, there is available only a very substantial over-strength pipe for the particular requirement, and therefore the engineer is forced to specify an unnecessarily expensive product.
While the above noted publication refers to lock seam pipe, with helical corrugations, and to resistance spot-welded pipe with circumferential seams, there has recently been introduced equipment for producing spiral wound butt-welded pipes. This equipment is manufactured by Armco Steel Company. By avoiding the rolled edges which produce a lock seam, and utilizing the butt-welded seam, a reduction of metal cost of approximately five percent is achieved. The equipment required to produce the butt-welded seam of the corrugated helical steel pipe is expensive, and therefore, although there is a saving in direct metal cost, there is a substantial addition in overall cost due to the noted welding equipment.